In chemical recovery plants associated with the production of paper pulp, direct contact evaporators (cascade evaporators) are sometimes associated with the recovery boiler/furnace. In such systems, the black liquor from the pulping process is oxidized and is fed at approximately a 50% total solids concentration to the cascade evaporator. In the evaporator its solids concentration is increased to about 65% (the increase in solids content is limited by the high viscosity of the oxidized black liquor), and then it is fed to the recovery boiler as fuel. This commercial system has a number of disadvantages, including:
The solids concentration of the black liquor when fed to the boiler is limited due to high viscosity; PA1 The oxidizing process causes a reduction of the fuel value of the black liquor by wet oxidizing Na.sub.2 S to Na.sub.2 SO.sub.4 ; and PA1 There is the potential for sulphur off gases at the cascade evaporator because of the presence of significant amounts of particular sulfur containing chemicals in the black liquor at that stage. This last problem has caused some installations to replace direct contact evaporators that were otherwise properly functioning since it is difficult to remove the sulfur off gases generated at this particular location.
According to the present invention, a method and apparatus are provided for treating black liquor associated with direct contact evaporators to overcome the drawbacks, and solve the problems, set forth above. According to the present invention, it is possible to increase the solids content of the black liquor up to about 80%. Sulfur is removed as H.sub.2 S so there is no need to oxidize the black liquor, thereby eliminating the loss of its heating value caused by oxidation, and minimizing the sulfur compounds that reach the direct contact evaporator. Further, the sulfur that is removed can be used in other beneficial ways within the pulp plant.
Some of the more basic aspects of the present invention are the heating of the black liquor--such as disclosed in U. S. Pat. No. 4,929,307 (the disclosure of which is incorporated by reference herein)--to drive off organic sulfur containing gases, such as DMS; removing the sulfur containing compounds from the off gases and using them in the pulp mill--such as disclosed in copending U.S. application Ser. No. 07/651,462, filed Feb. 6, 1991 now abandoned (the disclosure of which is incorporated by reference herein); and passing the black liquor (after heat treatment) into stripping contact with flue gases from the recovery boiler.
According to one aspect of the present invention there is provided a method of treating black liquor from a cellulose pulp producing facility, using a direct contactor evaporator and a recovery boiler having a flue gas discharge, comprising the following steps: (a) Heating black liquor at a temperature and for a time period sufficient to drive the majority of the sulfur containing organic compound gases from the black liquor. (b) Removing substantially the remaining sulfur in the black liquor by utilizing flue gases from the boiler flue gas discharge. (c) Passing the hot flue gases through the direct contact evaporator. (d) After step (b), feeding the black liquor to the direct contact evaporator to increase the solids content thereof; and (e) feeding the increased solids content black liquor to the recovery boiler to effect combustion thereof and produce hot flue gases. Desirably, the flue gases for step (b) are removed from the flue gas discharge at a point of withdrawal; and the method comprises the further step (f) of cleaning the removed flue gases after step (b) to remove the sulfur therefrom, and then returning the cleaned gases to the flue discharge downstream of the point of withdrawal. Also, preferably step (b) is practiced by bringing flue gases from the flue gas discharge, from a point downstream of the direct contact evaporator, into contact with the black liquor.
According to another aspect of the present invention there is provided a method of treating black liquor from a cellulose pulp producing facility, using a direct contactor evaporator and a recovery boiler having a flue gas discharge, comprising the following steps: (a) Heating black liquor at a temperature and for a time period sufficient to drive tile majority of the sulfur containing organic compound gases from the black liquor. (b) Passing the hot flue gases through the direct contact evaporator. (c) After step (a), feeding the black liquor to the direct contact evaporator to increase the solids content thereof. (d) Feeding the increased solids content black liquor to the recovery boiler to effect combustion thereof and produce hot flue gases; and (e) treating the gases with sulfur containing organic compounds to produce sulfur containing chemicals useful in thee pulping of cellulose in the cellulose pulp producing facility. The method also preferably comprises the further step (f), between steps (a) and (c), of stripping off the remaining sulphur from the black liquor, by passing it into contact with flue gases from the recovery boiler.
In the practice of the method of the invention, there may also be the further step of increasing the solids content of the black liquor, as by feeding at least a portion thereof to another evaporator (such as a falling film evaporator) to increase the solids content thereof before it is fed to the cascade evaporator. Normally, the black liquor in step (a) has a solids content of roughly 50%, which is increased to roughly 75-80% prior to feed to the recovery boiler. Step (e) may be practiced by converting the off gases to primarily methane and hydrogen sulfide, and then separating the methane and hydrogen sulfide, e.g. by passing the converted off gases into contact with white liquor from the pulp processing facility, and then using the enhanced sulfidity white liquor in the pulp processing facility.
According to another aspect of the present invention, there is a method of treating black liquor according to the following steps: (a) Stripping off sulphur containing compounds from the black liquor. (b) After step (a), feeding the black liquor to the direct contact evaporator to increase the solids content thereof. ( c ) Feeding the increased solids content black liquor to the recovery boiler to effect combustion thereof and produce hot flue gases; and (d) passing the hot flue gases through the direct contact evaporator. Step (a) is preferably practiced by bringing flue gases from the recovery boiler, from a point downstream of the direct contact evaporator, into contact within the black liquor.
According to yet another aspect of the present invention, apparatus for treating black liquor in a cellulose pulp processing facility is provided. The apparatus comprises the following structures: A recovery boiler having a black liquor inlet and a flue gas outlet, for combusting black liquor fed to the inlet to produce flue gases. A direct contact evaporator. A first conduit leading from the flue gas outlet to the direct contact evaporator, and a second conduit leading downstream from the evaporator. Means for bringing black liquor into stripping contact with hot gases. Means for feeding flue gases from the second conduit to the means for bringing black liquor into stripping contact with hot gases. Means for feeding black liquor to said means for bringing black liquor into stripping contact with hot gases; and means for feeding black liquor from the means for bringing black liquor into stripping contact with hot gases to the direct contact evaporator.
Means for increasing the solids consistency of black liquor (such as a falling film evaporator) may be disposed in the means for feeding black liquor from the means for bringing black liquor into stripping contact with hot gases to the direct contact evaporator. The means for feeding flue gases from the second conduit to the means for bringing black liquor into stripping contact with hot gases may comprise a conduit and a fan in the conduit. There may further be means for heating black liquor to drive off the majority of the organic sulphur compounds therefrom, the heating means connected to the means for feeding black liquor to the means for bringing black liquor into stripping contact with hot gases.
It is the primary object of the present invention to provide for the treatment of black liquor with a cascade evaporator to maximize the fuel value of the black liquor, minimize the possibility of significant sulfur emissions from the evaporator, and/or utilize sulfur from the black liquor in advantageous manners in other parts of the facility. This and other objects of the invention will become clear from the detailed description of the drawings, and from the appended claims.